It has long been known that coal can be used to produce gaseous fuel as a result of various chemical reactions. The destructive distillation of bituminous coal produces highly volatile and combustible compounds which are principally hydrocarbons. Water gas, a mixture of carbon monoxide and hydrogen, is made by passing superheated steam over hot carbon (coal). Produce gas, carbon monoxide, is the product of the incomplete combustion of carbon and is produced by passing air/oxygen over hot coal.
In the well-known, currently used coal gasification processes, the various chemical reactions usually take place in the presence of fluidized catalyst and the reaction products are further reacted, as in a methanation reaction, to upgrade the reaction products. Typically, the initial reactants are carbon (coal), steam and oxygen. Where oxygen is supplied as air, it is desirable to remove the coal gasification tail gas from the reaction products. The tail gas is primarily nitrogen and, since its temperature can exceed 1800.degree. F., it is desirable to recover the temperature and pressure therefrom. The high temperature of the tail gas presents difficulties in energy recovery in that an expander wheel cannot maintain its working strength at these temperatures without cooling or other thermal protection. Thermally protecting a rotating member presents great difficulties where cooling of the motive fluid is to be minimized in order to permit greater energy recovery. Additionally, although turbine blades can be made hollow, i.e. with cooling holes, impeller blades cannot.